4K boxes - difference in audio settings

1 have the Xi6 Wireless box as well as a wired DVR box (XG1v4) - and I noticed something very interesting I am hoping others can explain, or one of our Xfinity experts can address.

I have a Yamaha receiver (A6A) that is their latest model, full surround sound and can read out exactly what is being sent to it.

On the wired DVR 4k box, there are 3 settings to choose from

1. Stereo - sent out as 2.0 PCM
2. Auto detect - sent out as 5.1 Dolby Digital
3. Expert - sent out as 5.1 Dolby Digital Plus

A couple of months ago, I got a 2nd 4K box and got one of those wireless Xi6 boxes, very cool that it does Dolby vision (the XG1v4 does not do Dolby Vision) - but something didnt quite sound the same, so I did the same thing as above and found that:

1. Stereo - sent out as 2.0 PCM - the same as above
2. Auto detect - sent out at 5.1 PCM - this is different than above and causes something interesting (see comment below)
3. Expert - sent out at 5.1 Dolby Digital Plus - same as above.

Not that it REALLY matters to me because I'll just keep it on expert mode moving forward, but others should be aware of this difference.

The issue I referenced above:

1. My receiver when getting a 5.1 Dolby signal processes it up to 7.1, and my surround back channels are engaged, a great full enveloping surround sound experience. This is true for 10+ years even when I only had the HD boxes.
2. When my receiver gets a 5.1 PCM signal, it does NOT process it up to 7.1, and that surround sound was just less enveloping.

I contacted Yamaha and all my setting are correct on the receiver. My question is, why does the Xi6, even when it says "Dolby Digital" - in the onscreen menu for auto detect, send a PCM signal out of the box and to my receiver?

One would think there would be consistencies across the board for these kinds of settings.

Any other customers have an idea?

Any Xfinity employees have any idea, or wish to have me start a chat so it can be discussed further?

Thank you for your time and attention to this